Hot Work on Ammonia Systems

Work Place notice boards together with discussion at your next company safety meeting.

A recent occurrence has highlighted the need to remind all technical and site management staff of the potential dangers of carrying out hot work on a vessel or on pipe work that has previously contained ammonia. A very small ammonia explosion inside a pipe happened on an industrial contract installation where we are modifying an existing ammonia system. The explosion blew out the molten weld pool as a welder was carrying out a weld while fitting a new level column to an existing vessel. Although no one was hurt there is a potential for harm to personnel and equipment in similar situations.

What is the danger?

Ammonia is explosive at concentrations between 15 and 30% by volume in air. Although this is a very high concentration in terms of the maximum ammonia concentration people could stay in (200 ppm or 0.0002%) it is possible to reach the lower explosive limit if the system is enclosed and ammonia liquid is continuing to boil off from a pool in a vessel or piping, vaporise from oil in the bottom of a vessel or leak into the section from a live pipe though passing valves. Please remember that, due to the high latent heat, it is five times more difficult to boil off liquid ammonia than most other refrigerants. This means that ammonia liquid can remain in the bottom of a vessel or pipe (particularly if it is insulated) even after repeated pumping down to very low pressures. It is difficult to ignite ammonia and then it burns slowly relative to hydrocarbons and “popping” of air / ammonia mixtures during hot work is not unknown but it is essential that we manage the risk.

How can we prevent this?

Ignition can be prevented by ensuring the ammonia concentration near the hot work cannot reach anywhere near the lower explosive limit as the hot work is being carried out. This will require good forced ventilation of the vessel or pipe to be modified.

What can I do now?

Until new guidance is issued a risk assessment and method statement must be prepared for hot work on any pipe or vessel that has previously contained ammonia. Risk Assessments and Method Statements should be approved by higher management prior to commencement of any work. The review of the method statement will consider if the control measures are sufficient to ensure the system is sufficiently well ventilated to prevent a build-up of ammonia vapour in the area where the hot work is being carried out. It will also require good isolation (such as double block and bleed valves) from live sections of the system. In addition, as with all work on ammonia systems, reference must be made to Star Technical Bulletin 101 – Ammonia Working Practices.